Temperate Coniferous Forests Richard H Waring Volume 2, The Earth system: biological and ecological dimensions of global environmental change, pp 560–565 Edited by Professor Harold A Mooney and Dr Josep G Canadell in Encyclopedia of Global Environmental Change (ISBN 0-471-97796-9) Editor-in-Chief Ted Munn John Wiley & Sons, Ltd, Chichester, 2002 and sunken stomata and a large proportion of their stems Temperate Coniferous consists of sapwood that serves as a storage reservoir, as Forests well as a conduit for transport of water from roots to leaves. Richard H Waring GEOGRAPHIC DISTRIBUTION Oregon State University, Eugene, OR, USA Today, temperate evergreen coniferous forests cover app- roximately 2.4 ð 106 km2 (Melillo et al., 1993; Landsberg Temperate coniferous forests include representatives of the and Gower, 1997). Conifers dominate the montane forests most massive forms of terrestrial life on Earth. In the in North America, Europe, and China. Smaller areas of tem- coastal redwood (Sequoia sempervirens) forests of northern perate conifers are located in montane regions of Korea, California, trees may reach heights >100 m and accumu- Japan, Mexico, Nicaragua, and Guatemala. In Europe, the late biomass in excess of 3000 t ha1 . In the more extensive distribution of native and introduced conifers has been Douglas fir (Pseudotsuga menziesii) forests of the Pacific expanded into areas more climatically favorable to hard- Northwest, USA and Canada, biomass in older stands fre- woods. Pines (Pinus), represented by more than 90 species, quently reaches 1000 t ha1 , with an additional 500 t ha1 are the most widely distributed conifer and their range has of standing dead and fallen trees. Above-ground biomass increased by widespread planting in the Southern Hemi- ranges widely, however, as does above-ground growth. sphere. Other important genera include the firs (Abies), Temperate coniferous forests produce an average of about spruces (Picea), hemlocks (Tsuga), false-hemlocks (Pseu- 5tha1 year1 of above-ground biomass but in climat- dotsuga), larches (Larix), cypresses (Cupressus), cedars ically diverse areas production may range from <1to (Chamaecyparis, Thuja, Libocedrus) and juniper (Junipe- >20 t ha1 year1 (Runyon et al., 1994). rus). In the yews (Taxus), the seeds are not borne in Because of their high growth capabilities, favorable wood cones but are enclosed within a fleshy structure. Addi- structural properties and columnar form, temperate conif- tional conifer genera (Araucaria, Fitzroya, Austrocedrus) erous forests play a disproportionate role in meeting global are present in the Southern Hemisphere, where they were demands for lumber and fiber. As a consequence, the geo- once more widely distributed (Axelrod et al., 1991). graphic range of temperate conifers has been extended beyond the native ranges of many species. In this contribution we focus on the attributes of tem- ECOLOGICAL DISTRIBUTION perate evergreen coniferous forests that distinguish them from their major competitors, deciduous hardwoods. This The potential ecological niche of temperate conifers falls differentiation should help us anticipate the future role and within regions of >250 mm in annual precipitation that experience subfreezing conditions, down to, but not below importance of temperate coniferous evergreen in relation to ° rising levels of air pollution, atmospheric carbon dioxide 45 C, which is the limit for supercooling of water. Only boreal tree species, which include some pines and spruces, (CO2 ), and changing climate. are adapted to temperatures below this limit (Waring and Running, 1998). In regions where precipitation is well EVOLUTION distributed throughout the growing season, deciduous hard- woods usually dominate over temperate conifers. Following Conifers bear naked seeds, unlike angiosperms, and dif- major disturbances, however, conifers can become estab- fer from other gymnosperms by having the seeds arranged lished and achieve temporary dominance (Landsberg and inside cones. During the Carboniferous period (300 million Gower, 1997). years ago), conifers represented a minor group of gym- In the Pacific Northwest region, where summer drought nosperms, but they evolved rapidly as conditions became is common, the situation is reversed and many long-lived drier and colder during the Permian (286–240 million years conifers replace earlier established hardwoods (Waring and ago). Temperate coniferous forests were most widely dis- Franklin, 1979). To explain these shifts in dominance tributed during the Tertiary period (90–15 million years requires an appreciation of how resources are captured by ago), with coastal redwood (Sequoia sempervirens)and temperate evergreen conifers throughout the year. dawn redwood (Metasequoia) distributed throughout both Eurasia and North America. Today, conifers are the domi- nant representatives of gymnosperms with about 50 genera PHYSIOLOGICAL AND MORPHOLOGICAL and 550 species, predominantly in the Northern Hemisphere ADAPTATIONS OF CONIFERS (Raven et al., 1986). Conifers are well adapted to arid and cold conditions. The Conifer leaves are more clumped and narrower in cross- surfaces of needle-shaped leaves are protected by waxes section than the foliage of most angiosperms, and the 2 THE EARTH SYSTEM: BIOLOGICAL AND ECOLOGICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE branch structure is much less extensive. These proper- The evolutionary advantages that conifers have on infer- ties allow light to penetrate more deeply through conifer tile soils can place them at a disadvantage on more fertile canopies than through hardwood forests with comparable sites. On infertile soils, most available nitrogen (N) is in C1 leaf area. As a result, the maximum surface area of conifer the form of ammonium (NH4 ), which conifers take up 2 1 foliage, often expressed as m of (projected) leaf area per selectively in preference to nitrate–nitrogen (NO3 ). As m2 of ground surface, may reach 10–12, while hardwood nitrate–nitrogen becomes more available, conifers are dis- forests rarely exceed half these values. In addition, conifer advantaged in competition with hardwoods (Smirnoff and canopies reflect only about 5–10% of intercepted solar radi- Stewart, 1985). ation, whereas hardwoods reflect 15–25%. In combination, these differences in canopy properties permit coniferous forests to absorb a greater proportion of incoming solar ECOSYSTEM RESPONSES OF TEMPERATE radiation than forests composed of hardwoods (Jones 1992). CONIFEROUS FORESTS Although conifers absorb a larger fraction of intercepted Decomposition radiation than hardwoods, they are less prone to suffer damage from elevated temperatures because the needle- Leaf litter, wood, and root materials produced by ever- shaped foliage dissipates heat efficiently, even under unven- green conifers usually contain twice the amount of carbon tilated conditions. Consequently, conifers conserve water (C) in relation to nitrogen found in corresponding materi- under conditions where broad-leaf species must transpire at als produced by deciduous angiosperms. As a result, the higher rates, wilt, or shed foliage to prevent temperatures decomposition of coniferous litter is usually 3–4 times from rising above 45 °C, when enzymes begin to become slower than hardwood litter, leading to a greater accumu- unstable. lation of forest floor litter under conifers (Figure 1). Another foliage characteristic that distinguishes most With time, as litter decays, soils under coniferous forests conifers from other evergreen vegetation is the duration maintain high C–N ratios and serve as storage sites for that leaves remain functional. Conifers generally hold some amounts of carbon that far exceed above-ground biomass, foliage for 3–5 years, although a few species maintain and have turnover times of centuries and millennia. This functional leaves for 30–40 years (Landsberg and Gower, 1997). The fact that temperate evergreen conifers can main- 100 tain dense, needle-leaf canopies, composed of multiple-age MRT = 20 10 classes of foliage, affects many ecosystem processes includ- ) 80 ing the capture of water, nutrients, and pollutants from the 1 − air, and the rates of decomposition, nutrient release, and 1 soil organic matter accumulation. 60 Anatomically, long-lived foliage is characterized by an 4 epidermis covered with a thick waxy cuticle, above one 2 3 or more layers of compactly arranged, thick-walled cells, 40 which in turn surround a structurally reinforced vascular system. The result is that nutrient concentrations in conifer Forest floor mass (t ha Forest 4 2 needles may be only half those in deciduous hardwood 20 5 leaves of equivalent mass. Lower nutrient concentrations 6 in individual leaves translate into lower demand, yet long- 7 lived foliage provides a greater store of nutrients available 0 24681012 for transfer during peak growth periods than is available in Litterfall (t ha−1 year −1) deciduous hardwoods. In addition, in environments where mineral weather- Figure 1 Average forest floor mass plotted against lit- ing rates are slow and nutrient availability limited, ever- ter-fall for (1) boreal needle-leaved evergreens, (2) boreal green conifers can take up nutrients from the soil as long broad-leaved deciduous, (3) temperate needle-leaved ever- green, (4) temperate broad-leaved evergreen, (5) tem- as conditions remain above freezing. Photosynthesis can perate broad-leaved deciduous, (6) tropical broad-leaved also continue under these conditions. As a result of nutri- evergreen, and (7) tropical